The field of application of vibration-resistant playback devices applies, in particular, to the portable and mobile use of playback devices of optical information media, in which uninterrupted and undisturbed reproduction of information must be ensured despite vibrations, the field of application not being restricted, however, to optical information media.
Optical scanning devices, in particular, are sensitive to mechanical vibrations, since the optical scanning system can be pitched from the track by vibrations and, as a result, the read operation of the information stored on the information medium is interrupted. During stationary operation, these vibrations are so small that they can be adequately damped by means of mechanical precautions and electrical control loops. However, in the case of mobile use, such as, for example, in a car, or in the case of portable devices, the abovementioned measures no longer suffice, and use is made of a buffer which buffers the data stream for the purpose of reproduction during an interruption of the reading of the information medium. A buffer of this type is known in accordance with the data sheet of the CXD2511Q/R circuit from Sony (Vibration-Protective Memory Controller for CD Players). In order to provide a data reserve, the data are read from the CD at twice the speed and are stored in the buffer. At the same time, data are read at normal speed from the buffer for the purpose of reproduction. In order to prevent an overflow of the buffer, the reading of the information medium or the writing of data to the buffer must be continuously interrupted and the read operation must be resumed after the occupancy of the buffer has fallen below a minimum occupancy. For this purpose, the scanning or reading system must be returned to the location of the interruption and the reading of the information medium must be continuously resumed. This sequential pattern of events is continued unremittingly provided that no vibration occurs. As a result, a high number of resets or returns of the scanning or reading system are already necessary during vibration-free operation.
If a vibration occurs during a reading pause, no special measures are necessary.
However, if the vibration occurs during the read operation, then the filling of the buffer must be stopped immediately and the scanning system must be returned as quickly as possible to the location at which the read operation was interrupted, in order to read anew data from the information medium or the CD. Following successful conclusion of this operation, the filling of the buffer is resumed. Consequently, this jumping or resetting of the scanning system to the location of the interruption of the reading of the CD is not only necessary when vibrations occur but also necessary when vibrations do not occur in the vibration resistance mode of operation, since the read operation which is carried out at twice the speed must be interrupted when the buffer is full.
In order to write data from the information medium to the buffer following an interruption in accordance with the order present on the information medium, the reading of the CD is resumed using data prior to the last subcode address evaluated as valid by a microprocessor, and in order to start the writing to the buffer, or for the sequentially correct resumption, a comparison between the stored and the read audio data is carried out. This audio data comparison, which controls the resumption of the writing to the buffer in the event of a positive result, is carried out after the decoding and error correction and directly before the digital/analog conversion. The buffer is connected to the de-interleaving and error correction unit of the decoder and is arranged directly before the digital/analog converter. (Interleaving=in German! Verschachtelung, Uberlappung overlapping!, Codespreizung code spreading!).
In addition to the relatively high number of returns when no vibrations occur, a problem of this solution is the reliability of the data synchronization for starting the write operation to the buffer. In the case of the known solution, the purpose of synchronization following an interruption of the writing to the buffer is served by using a predetermined volume of data to find, by comparison, the identical location of the data which have already been stored in the buffer. The synchronization which is based on the audio data comparison is not always successful, since, owing to errors, the data bits do not always appear in the same way and accurately timed. On account of the error rate, it is not always possible to derive the same data from the identical location on the information medium. A chronological offset also occurs when comparing very soft passages, in which only zeros are compared, or when comparing passages which contain multiple repetitions such as, for example, a sinusoidal tone at a specific frequency.
The data comparison which must be carried out frequently in connection with the interruptions does not guarantee the determination of the exact synchronization instant, leads to jumps in the audio data and consequently, owing to chronological jumps in the title or disk time, does not ensure accurately timed reproduction corresponding to the stored information.
Therefore, a comparison with stepped accuracy or a comparison in a plurality of stages is already carried out in the known solution in order, if possible, to bring about correspondence. The accuracy and the length of the comparison are varied in order to permit tolerances during the comparison. This results in jumps in the audio data and, consequently, does not ensure accurately timed reproduction corresponding to the stored information. The more or less exact synchronization takes place in the buffer arranged directly upstream of the digital/analog converter, with the result that erroneous synchronization causes audible interference.
Moreover, a DRAM must be provided as the buffer for the most exact data comparison possible, which DRAM represents a considerable cost factor.